Electronic control modules are often used to control the output of an internal combustion engine. A typical control scheme used by such a module might involve both closed loop engine speed control and closed loop rack position control within the engine speed control loop. A known electronic control module is disclosed in U.S. Pat. No. 4,368,705, issued to Stevenson et. al. on Jan. 18, 1983.
In a control module such as disclosed in U.S. Pat. No. 4,368,705, a throttle position signal, which corresponds to a desired engine speed, is provided as a command input. An engine speed sensor is located on the engine and produces a feedback signal to the control module. An engine speed error signal is calculated as the difference between the actual engine speed feedback signal and the throttle signal. The error signal is input to an engine speed controller which calculates a desired rack position signal and inputs that command to the rack position loop.
Known control modules often place the rack position loop within the engine speed loop. The desired rack position signal produced by the engine speed control loop is an input command to the rack position loop. A position sensor is located on the rack which produces a rack position feedback signal. A rack position error signal is calculated as the difference between the desired rack position command and the actual rack position. The rack position error is an input to the rack controller which in turn develops an actuator signal that is delivered to the rack actuator motor to drive the rack position error signal to zero (i.e., so that the actual rack position equals the desired rack position).
The engine speed feedback signal is critical in closed loop engine speed control systems. If the engine speed sensor fails or the electrical connection between the sensor and the controller is compromised, no engine speed feedback signal will be present at the controller. Then, because there is no actual engine speed signal to subtract from the desired engine speed command, the engine speed error signal will equal the throttle command and it will be impossible for the controller to drive the engine speed error signal to zero. The controller will continue to command increased output from the engine regardless of the engine's actual speed. The engine will eventually exceed its maximum operating speed and break down. In most closed loop engine speed controls, to prevent engine damage, the controller will prevent the engine from operating when the engine speed feedback signal is not present.
However, completely shutting an engine down may be undesirable in some applications. For example, in a marine application, leaving a boat without power prevents it from maneuvering according to surface conditions, weather, etc. The present invention is directed toward overcoming one or more of these problems.